1. Field of the Invention
The invention relates to the fabrication of Integrated Circuit devices and more specifically to the formation of air gaps as a low dielectric constant material between conductor lines on the same or on different levels.
2. Description of the Prior Art
The formation of air gaps between conducting lines of high speed Integrated Circuits (IC's) is typically a combination of the deposition of a metal layer, selective etching of the metal layer to form the desired line patterns, the deposition of a porous dielectric layer or a disposable liquid layer which is then selectively removed to form the desired air-gaps.
The continuing effort to reduce the size of individual transistors and other devices commonly integrated on a semiconductor chip and to increase the density of Integrated Circuits results in a continuing reduction of the separation between conducting layers of materials. This reduction results in an increase of capacitive crosstalk between adjacent conductor lines of a semiconductor circuit, that is the voltage on the first conductor line alters or affects the voltage on the second conductor line. This alteration in voltage can cause erroneous voltage levels in the Integrated Circuit making the IC increasingly prone to faulty operation. It becomes therefore imperative to reduce the resistance capacitance (RC) time constant and the crosstalk between adjacent conducting lines.
The capacitance between adjacent conducting lines is highly dependent on the insulator or dielectric used to separate the conducting lines. Conventional semiconductor fabrication typically uses silicon dioxide as a dielectric; this has a dielectric constant of about 3.9.
The use of many of the low dielectric constant materials is not feasible due to the fact that equipment is not available to properly process the new dielectric material in various integrated circuits. Also, the chemical or physical properties of many low dielectric constant materials are usually difficult to make compatible with or integrate into conventional integrated circuit processing.
The lowest possible and therefore the ideal dielectric constant is 1.0, this is the dielectric constant of a vacuum whereas air has a dielectric constant of slightly larger than 1.0.
To reduce said capacitive coupling and reduce the capacitive crosstalk, a major objective in the design of IC's is to reduce the dielectric constant (k) of the insulating layer between adjacent conductor lines of semiconductor circuits. The present invention makes a significant contribution within the scope of this effort.
U.S. Pat. No. 5,461,003 (Haveman et al.) shows a method of forming air gaps between metal lines by: 1) forming a photoresist layer (disposable layer) 18 (see table 1) between and over metal lines; 2) forming a low-k dielectric (HSQ) (20 see table 1) (but not by having openings thereover); 3) removing the photoresist 18 and thereby forming air gaps and 4) by forming a capping oxide layer to close up the air gaps. However, this patent differs from the present invention in step 2).
U.S. Pat. No. 5,750,415 (Gnade et al.) U.S. Pat. No. 5,792,706 (Michael et al.) and U.S. Pat. No. 5,407,860 (Stotz et al.) show air gap processes.